1. Field of the Invention
The present invention relates to communications networks in general and more particularly to interconnected communications networks.
2. Prior Art
It is well known in the prior art to interconnect a plurality of stations to allow them to communicate with one another. If the configuration is limited to a relatively small geographical area (such as a building, college campus, etc.), it is called a Local Area Network (LAN). A typical LAN includes a communications media interconnecting a number of stations. For maximum efficiency, each LAN has a maximum number of stations which it can support. If the number of stations exceeds the maximum limit, the efficiency of the LAN goes down. Depending on the use and/or configuration of the LAN, there are times when the load (number of stations) has to be maintained below the maximum limit. In other words, there are times when the number of stations are fewer than the maximum number allowed for that particular LAN.
Usually, if the total number of stations cannot be supported on a single LAN, additional LANS are provided in the same geographical area. It is known that such LANs may themselves be interconnected by interconnecting devices sometimes called "bridges" or "gateways", etc. The interconnected LANS form a unified or combined network in which stations on one LAN may communicate with stations on other LANS. The interconnecting devices are usually of the store-and-forward type which forwards messages received from one LAN onto the other.
The prior art provides two bridging architectures which can be used to interconnect LANs. The two architectures are "Source Routing" and "Transparent Bridging". Both prior art architectures and related protocols are fully described in the literature to be identified hereafter. Therefore, only a short description of each of the two architectures will be given here in order to identify the environment in which applicants' invention is used.
In the "Source Routing" architecture and related protocols, the specific route which a frame must traverse between one end station called the "Source" and another end station called the "Destination" is carried in the frame. Prior to issuing the frame with routing information, the "source station" issues an "explorer frame" which is propagated throughout the network to the destination station. As the explorer frame traverses the network, the bridge enters routing information such as bridge identification number, LAN number, etc. On arriving on the destination LAN, the explorer frame is copied by the destination station and is rerouted via the specific route to the origination station which stores the route information and uses it to send subsequent messages to the destination station. A more detailed description of the source routing architecture and protocol is set forth in various IEEE 802.1 and IEEE 802.5 draft standards for MAC bridges and Token Ring LANs and in an article entitled, "Table-Free Bridging" by Daniel Pitt and Jacalyn L. Winkler, (IEEE Journal on Selected Areas in Communication, Vol. SAC-5, No. 9, December, 1987, pp. 1454-1462).
In the transparent bridging architecture and related protocols, the routing information is not carried in the frame. Instead, the source address in each frame and address tables in the interconnecting bridge are used to forward the frame from one LAN to another. In particular, the addresses of stations which are connected to respective bridged LANs are recorded and maintained in separate tables which are generated in the interconnecting bridge. When a source station on a first LAN wishes to communicate with a destination station located on another LAN, the destination address of the destination station is included in the frame. The interconnection bridge, on receiving the frame, compares the destination address with address entries in the address table which is associated with the first LAN. If a match is not made and the bridge is in a "forwarding state", the frame is forwarded to the other LAN. A more detailed description of the transparent bridge architecture and related protocols are set forth in International Standard ISO 10038 (IEEE Draft Standard P802.1d/D9). In addition, specific features and implementations of transparent bridging are set forth in U.S. Pat. Nos. 4,597,078 and 4,627,052.
Even though the "source routing" and transparent bridge architectures work well in their respective environments, they are incompatible and could not be used in a bridged heterogeneous communications network. Stated another way, communications networks which use different bridging architectures could not be easily interconnected. This causes unnecessary restrictions and hardships on users who may elect to interconnect communications networks which use different bridging architectures and protocols.